Cartridges with a piston or plunger are well-known in insulin pump systems whereby detection of the fill level of the cartridge is important to ensure that the pump system functions correctly and/or that the correct dosage is supplied and/or to signal replacement of the cartridge.
Various cartridge fill level detection systems have been proposed as described in U.S. Pat. No. 5,704,922, U.S. Pat. No. 6,352,523, WO 2007/107558, and DE 102004040441.
In U.S. Pat. No. 5,704,922, a syringe having electrical contacts points for metering doses is disclosed. A disadvantage of this system is the risk of short-circuiting between contacts leading to a false fill level reading. Moreover, in view of the difficulty of providing a large number of contacts, the fill level accuracy of the system is poor. The need to connect the contacts on the plunger requires a connector that is costly and voluminous.
Some of these drawbacks are overcome by contactless fill level detection systems as described for example in U.S. Pat. No. 6,352,523, DE 102004040441 or WO 2007/107558. In these known systems, electrodes are positioned on either side of the cartridge to measure the capacitance which is dependent, inter alia, on the dielectric properties between the electrodes. The dielectric properties depend on the material forming the cartridge walls, the distance separating the electrodes, the material of the plunger, the dielectric value of the liquid in the cartridge and the fill level. As the liquid in the cartridge has a dielectric properties that are different to air and the material of the plunger, the overall capacitance value will vary as the plunger advances and liquid is expelled out of the cartridge. One of the major drawbacks of this measuring system is its sensitivity to interference, and the need to accurately calibrate the system in view of the reliance on many factors affecting the capacitance measurement. The accuracy and reliability of such a system may in many applications be insufficient. Also, the need to adjust calculation of the measurement and to calibrate the cartridge fill level detection as a function of the diameter of the cartridge and the material used for the cartridge as well as the properties of the liquid in the cartridge, is costly.